Package _py3k

Python 3 compatibility tools (PRIVATE).
We used to have lines like this under Python 2 in order to use
iterator based zip, map and filter (in Python 3 these functions
are all iterator based)::
from future_builtins import zip
There is no similar option for range yet, other than::
range = xrange
input = raw_input
or::
from __builtin__ import xrange as range
from __builtin__ import raw_input as input
Under Python 3 these imports need to be removed. Also, deliberate
importing of built in functions like open changes from Python 2::
from __builtin__ import open
to this under Python 3::
from builtins import open
Instead, we can do this under either Python 2 or 3::
from Bio._py3k import open
from Bio._py3k import zip
Once we drop support for Python 2, the whole of Bio._py3k will
go away.